Potential induced degradation of pre-stressed photovoltaic modules: Effect of glass surface conductivity disruption

Abstract

Potential induced degradation (PID) due to high system voltages is considered as one of the possible degradation mechanisms of PV modules in the field. In the previous studies carried out at ASU-PRL, the surface conductivity of the entire glass was obtained using either conductive carbon layer (covering the entire glass surface and extending it to the frame) or humidity inside an environmental chamber. This study investigates the influence of disruption of glass surface conductivity on the PID. In this study, the conductive carbon layer was applied on the module's glass surface but without extending it to the frame and hence the surface conductivity was disrupted (no carbon layer) at 2 cm distance from the periphery of frame's inner edges. This study was carried out on the modules of different manufacturers under dry heat conditions at multiple stress temperatures and voltages. To replicate closeness to the field-aged modules, half of the selected modules for the PID investigation were pre-stressed under damp heat for 1000 hours and the other half under thermal cycling for 200 cycles. When the surface continuity was disrupted, the degradation was found to be absent or negligibly small even after 35 hours of negative bias at elevated temperatures. This preliminary study appears to indicate that the modules could become immune to PID losses if the continuity of the glass surface conductivity is disrupted at the inside boundary of the frame. The surface conductivity of the glass, due to water layer formation in a humid condition, close to the frame could be disrupted just by applying a transparent hydrophobic layer near the inner edges of the frame or by attaching the frameless laminate with the conductivity disrupting mounting methods such as glue-on rail on the backsheet.